1. Technical Field
Methods for forming capacitor of ferroelectric random access memory (hereinafter referred to as xe2x80x9cFeRAMxe2x80x9d) are disclosed. The disclosed methods can prevent the step difference resulting from an etch-back process and scratch on a Pt layer in a CMP process using a basic slurry by performing a CMP process using an acidic slurry including an organic acid when a storage electrode is isolated in a formation process of a FeRAM capacitor.
2. Description of the Related Art
The FeRAM is one of the nonvolatile memory devices using polarization inversion and hysteresis characteristics of ferroelectric material. The FeRAM is an ideal memory that can store information in a powered-down state and has a high operating speed, large capacitance and low power consumption comparable to those of a DRAM.
The ferroelectric materials of FeRAM include SrBi2Ta2O9 (hereinafter, referred to as xe2x80x9cSBTxe2x80x9d), SrxBi2-y(TaiNbj)2O9-z (hereinafter, referred to as xe2x80x9cSBTNxe2x80x9d), Pb(ZrxTi1-x)O3 (hereinafter, referred to as xe2x80x9cPZTxe2x80x9d), SrTiO3 (hereinafter, referred to as xe2x80x9cSTxe2x80x9d) or Bi4-xLaxTi3O12 (hereinafter, referred to as xe2x80x9cBLTxe2x80x9d). The ferroelectric material has high dielectric constant that ranges from a few hundreds to a few thousands at room temperature and has two stabilized remnant polarization states. Therefore, the above-described ferroelectric material can be formed into thin films and applied to nonvolatile memory devices.
The nonvolatile memory device incorporating the ferroelectric thin film may adjust directions of polarization by directions of an impressed electric field. The polarization induced by the electric field still remains although the electric field is eliminated. As a result, digital signals 1 and 0 may be stored depending on the directions of remnant polarization.
Since the ferroelectric material is a crystal, the lower materials disposed beneath the crystalline ferromagnetic thin film are important in growth of thin films. Platinum group metals or oxides thereof (hereinafter, referred to as xe2x80x9cplatinum group metalxe2x80x9d) such as Pt, Ir, Ru, RuO2 or IrO2 are generally used for ferroelectric electrodes materials. However, the platinum group metals do not easily react with other compounds because they are hard and stable refractory metals. As a result, it is difficult to etch the platinum group metals. In order to overcome this problem, a dry etching method is used to etch the platinum group metals by using species such as radicals, ions and electrons in plasma formed using an inert gas such as Ar or He with the reactive gas.
To form a storage electrode, a bottom electrode, in a FeRAM, an electrode is formed of a platinum group metal, Pt, and an insulating film such as SiO2 is deposited thereon. Then, an oxide etch back process is performed to isolate the storage electrode.
When an oxide etch back process is performed until the Pt layer (the storage electrode) is exposed, it is possible to isolate the storage electrode but difficult to etch the Pt layer. As a result, the oxide is polished more than the Pt layer, thereby generating a step difference. In this case, a ferroelectric material is not uniformly deposited in a subsequent process, and characteristics of ferroelectric material are not properly embodied.
When step difference is heavy, a ferroelectric material is deposited on a glue layer, thereby increasing leakage current. Additionally, uniformity of the ferroelectric material is degraded due to the step difference, thereby generating cracks in a subsequent annealing process.
In order to solve the problem of the etch back process for isolating a storage electrode, a CMP process is performed on the insulating film and the Pt layer using a basic slurry. However, although the problem of step difference generated in the etch back process is solved by performing a CMP process using a basic slurry, the surface of Pt layer is severely scratched as shown in FIG. 2.
Methods for forming capacitor of FeRAM are disclosed where a storage electrode can be isolated without generation of step difference between a storage electrode and an insulating film as well as without generation of scratch on a Pt layer which is a storage electrode material.